7648
I.N.Lykakis, M.Orfanopoulos / Tetrahedron Letters 45 (2004) 7645–7649
CH
3
H
CH
and molecular oxygen is a convenient and mild transfor-
mation forming mainly the corresponding tertiary
hydroperoxides and alcohols. The reasonable to high
chemical yields, the small amount of the C–C bond
cleavage products present, and the easy reduction of
the hydroperoxides to the corresponding alcohols, ren-
der the title reaction synthetically useful.
wO
3
Ar
Ar
Ar
CH
3
CH
3
RI1
O
2
CH
3
OO
CH
RI2
3
Et
3
SiH
decomposition
Acknowledgements
We thank the Greek Secretariat of Research and Tech-
nology (IIENED 2001 and Greek–French collaborative
research program Platon 2001) for financial support and
graduate fellowships to I. Lykakis. We also thank Dr. T.
Montangon for valuable comments and discussions.
CH
OOH
CH
3
CH
3
O
decomposition
O
Ar
Ar
Ar
-CH
3
CH
3
3
CH3
RI3
c
a
minor product
major product
3
Et SiH
CH
OH
CH
3
Ar
References and notes
3
b
1
. (a) Activation and Functionalization of Alkanes; Hill, C. L.,
Ed.; Wiley: New York, 1989; (b) Selective Hydrocarbons
Activation: Principles and Progress; Davies, J. A., Watson,
P. L., Greenberg, A., Liebman, J. F., Eds.; VCH: New
York, 1990; (c) The Chemistry of Alkanes and Cycloalk-
anes; Patai, S., Rappoport, Z., Eds.; John Wiley and Sons:
Chichester, 1992; (d) Metal-catalyzed Oxidation of
Organic Compounds; Sheldon, R. A., Kochi, J. K., Eds.;
Academic: New York, 1981.
3
Scheme 3. Proposed mechanism of the decatungstate/Et SiH system
oxidation of aryl alkanes.
derived from substrate 7 where the R substituent is a
1
small and freely rotating methyl group.
9c
It is interesting to note that lower conversions were also
observed in the oxidations of 4-nitro and 4-trifluoro-
methylphenyl propanes (Table 1, substrates 1 and 2).
These results support the proposal that the electron
delocalization of the cumyl radical intermediate RI1
2
3
. (a) Maldotti, A.; Molinari, A.; Amadelli, R. Chem.Rev.
2002, 102, 3811–3836; (b) Mizuno, N.; Misono, M. Chem.
Rev. 1998, 98, 199.
. A special issue of Chemical Reviews is devoted to
polyoxometalates: (a) Hill, C. L., Ed. Chem. Rev. 1998,
98, 1; (b) Hill, C. L.; Christina, M.; Prosser-McCartha, C.
M. Coord.Chem.Rev. 1995, 143, 407; (c) Papaconstan-
(
Scheme 3) by the electron-withdrawing group of the
para-phenyl substituent reduces the radicalÕs reactivity
towards molecular oxygen.
1
9b
tinou, E. Chem.Soc.Rev.
Mylonas, A.; Papaconstantinou, E. Chem.Soc.Rev. 2001,
0, 62.
. (a) Renneke, R. F.; Hill, C. L. Angew.Chem. 1988, 27,
526; (b) Renneke, R. F.; Pasquali, H.; Hill, C. L. J.Am.
1989, 18, 1; (d) Hiskia, A.;
3
A plausible mechanism that may rationalize these results
is shown in Scheme 3. In the first step, under the influ-
ence of irradiation, the decatungstate anion converts
to the relatively long-lived intermediate wO, which is
the species responsible for the C–H bond cleavage,
which forms the corresponding tertiary radical interme-
4
5
1
Chem.Soc. 1990, 112, 6585; (c) Combs-Walker, L. A.;
Hill, C. L. J.Am.Chem.Soc. 1992, 114, 938.
8
,9
. (a) Renneke, R. F.; Hill, C. L. J.Am.Chem.Soc.
1986,
108, 3528; (b) Prosser-McCartha, C. M.; Hill, C. L. J.Am.
Chem.Soc. 1990, 112, 3671.
diate RI . It is established that hydrogen abstraction
1
transfer (HAT) of this type takes place in the rate-deter-
6. (a) Giannotti, C.; Richter, C. Trends Photochem.Photo-
biol. 1997, 4, 43; (b) Maldotti, A.; Molinari, A.; Berga-
mini, P.; Amadelli, R.; Battioni, P.; Mansuy, D. J.Mol.
Catal.A 1996, 113, 147; (c) Maldotti, A.; Amadelli, R.;
Carassiti, V.; Molinari, A. Inorg.Chim.Acta 1997, 256,
8
,11,14
mining step of such reactions.
reduced species of decatungstate H W10
dized in the presence of molecular oxygen to give again
The single-electron-
þ
5ꢀ
is re-oxi-
32
O
4
32
ꢀ
8,9a
W O and a molecule of hydrogen peroxide.
In the
10
3
. Molinari, A.; Amadelli, R.; Mazzacani, A.; Sartori, G.;
09.
case of an electron transfer (ET) mechanism, a high
yield of the C–C bond cleavage products would be ex-
7
Maldotti, A. Langmuir 2002, 18, 5400.
8. Tanielian, C. Coord.Chem.Rev.
2
2
pected. The peroxyl and/or alkoxy intermediates (RI2
and RI ) can be trapped by Et SiH to form the tertiary
1998, 180, 1165.
3
3
9. (a) Tanielian, C.; Duffy, K.; Jones, A. J.Phys.Chem.B
1997, 101, 4276; (b) Tanielian, C.; Schweitzer, C.;
Seghrouchni, R.; Esch, M.; Mechin, R. Photochem.
Photobiol.Sci. 2003, 2, 297; (c) Tanielian, C.; Seghrouchni,
R.; Schweitzer, C. J.Phys.Chem.A 2003, 107, 1102.
0. (a) Duncan, D. C.; Netzel, T. L.; Hill, C. L. Inorg.Chem.
hydroperoxides and alcohols, respectively, as the major
1
6e
products. However in a recent study, the rate constant
of the reduction of cumylperoxyl radical with silanes
ꢀ
1 ꢀ1
was found to be in the range of 0.10–0.90M
low rate value is not consistent with a fast trapping of
s . This
1
1
1
995, 34, 4640; (b) Texier, I.; Delouis, J. F.; Delaire, J. A.;
Giannotti, C.; Plaza, P.; Martin, M. M. Chem.Phys.Lett.
999, 311, 139.
1. Duncan, D. C.; Fox, M. A. J.Phys.Chem.A 1998, 102,
559.
the peroxyl or alkoxyl radicals by Et SiH. Further study
3
is required in order to rationalize the exact role of Et3-
SiH in the title reaction.
1
4
In conclusion, the decatungstate photosensitized oxida-
tion of aryl alkanes in the presence of triethylsilane
12. Kothe, T.; Martschke, R.; Fischer, H. J.Chem.So c, .
Perkin Trans.2 1998, 503.